Handoff method for cellular digital mobile communication system and mobile station

ABSTRACT

According to a handoff method for a cellular digital mobile communication system, a communication test of a new channel is performed by utilizing a free time of a time-divisionally multiplexed channel. A mobile station of this invention includes a communication circuit for performing communication by using a time-divisionally multiplexed channel, a switching circuit for connecting a new channel during a free time of the time-divisionally multiplexed channel, and a communication test circuit for performing a communication test when the switching circuit is connected to the new channel.

BACKGROUND OF THE INVENTION

The present invention relates to a handoff (busy channel switching)method for a cellular digital mobile communication system constituted bysmall radio zones (cellular system).

A service area of the cellular system is divided into a large number ofradio zones (cell sites), and identical frequencies are utilized againin remote cell sites to increase the number of mobile telephone sets perfrequency. The number of potential users of mobile telephones is largerthan the actual number of users covered by the existing systems.Therefore, strong demand has arisen for a large-capacity system. Thesize of the cell site may be reduced to increase the system capacitywithin the limited frequency band. For example, if the radius of thecell site is reduced to 1/10, frequency utilization efficiency can beincreased to 100 times.

When the size of the cell site is minimized, however, the frequency ofcommunication channel switching, i.e., a handoff is increased since amobile station is moved from one site to another during communication. Ahandoff sequence of an existing system is described in Bell SystemTechnical Journal Vol. 58, No. 1, January 1979, P. 65. Upon designationof a new cell site channel, a mobile station interrupts communicationvia the present channel, and the channel is then switched to a newchannel. In this state, the mobile station performs a communication testand restarts communication on the new channel. According to this method,communication is interrupted for 0.7 to 0.8 sec during the communicationtest.

Since the communication is interrupted for 0.7 to 0.8 sec according tothe conventional handoff method, communication quality is greatlydegraded when the channels are frequently switched upon an increase inthe number of cell sites.

In addition, in the conventional handoff method, detection of movementof a mobile station between cell sites is concentrated on a base stationand a radio channel control station. When the size of the cell site isminimized, the handoff operation must be frequently performed, and acontrol amount is locally increased, thereby requiring a large-capacityapparatus.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a handoff methodsubstantially free from an interruption time and a mobile station forrealizing the method.

It is another object of the present invention to provide a handoffmethod in which a control amount is not locally concentrated, and amobile station for realizing this method.

In the handoff method of a cellular digital mobile communication systemaccording to the present invention, a free time of a time-divisionallymultiplexed channel is utilized to perform a communication test of a newchannel.

The mobile station of the present invention comprises communicatingmeans for performing communication using a time-divisionally multiplexedchannel, switching means for connecting a new channel during the freetime of the channel, and a communication test circuit for performing acommunication test of the new channel when the switching means switchesthe channel to the new channel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing allocation of cell sites of a cellular digitalmobile communication system which employs the present invention;

FIGS. 2A, 2B and 2C are views showing frame formats of channels of eachcell site according to an embodiment of a handoff method of the presentinvention;

FIG. 3 is a view showing a handoff sequence using the frame formatsshown in FIGS. 2A to 2C;

FIG. 4 is a view showing another handoff sequence using the frameformats shown in FIGS. 2A to 2C;

FIG. 5 is a block diagram of a mobile station for realizing the handoffsequences of FIGS. 3 and 4;

FIGS. 6A, 6B, 7A, and 7B are views showing frame formats of channelsaccording to another embodiment of the handoff method of the presentinvention;

FIG. 8 is a flow chart showing a handoff sequence using the channelframe formats shown in FIGS. 6A to 7B;

FIG. 9 is a flow chart showing another handoff sequence using thechannel frame formats shown in FIGS. 6A to 7B;

FIG. 10 is a block diagram showing a mobile station for realizing thehandoff sequences of FIGS. 8 and 9;

FIG. 11 is a view showing a system employing still another embodiment ofa handoff method of the present invention;

FIG. 12 is a view showing a time division operation of the carrierfrequency in the handoff method practiced by the system shown in FIG.11;

FIG. 13 is a view showing assignment of time-divisional carrierfrequencies shown in FIG. 12 to cell sites;

FIG. 14 is a view showing a handoff sequence executed according toassignment of the carrier frequencies shown in FIGS. 12 and 13;

FIG. 15 is a view showing another time division operation of carrierfrequencies in the handoff method practiced by the system shown in FIG.11;

FIG. 16 is a view showing assignment of the time-divisional carrierfrequencies (FIG. 15) to cell sites;

FIG. 17 is a block diagram showing a mobile communication systemaccording to still another embodiment of the present invention; and

FIG. 18 is a block diagram showing a detailed arrangement of a mobilestation shown in FIG. 17.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described withreference to the accompanying drawings.

FIG. 1 shows allocation of base stations of a cellular digital mobilecommunication system which employs a handoff method, i.e., a method ofswitching communication channels (to be referred to as channelshereinafter). Referring to FIG. 1, a coverage area of the systemconstitutes a grid consisting of radio zones, i.e., cell sites of alarge number of base stations 1. FIG. 2A shows a format assigned to abase station A, FIG. 2B shows a format assigned to base stations B, D,and F, and FIG. 2C shows a format assigned to base stations C, E and G.

In the channel format of the base station 1, a frame having a length of8 T (to be referred to as an 8 T frame hereinafter) constituted by timeslots each having a length of 2 T is repeated twice, and a frame havinga length of 4 T (to be referred to as a 4 T frame hereinafter) of timeslots each having a length of T are repeated eight times. When the frameformat is assigned to the base station 1 as described above, while the 8T frame is repeated twice, the adjacent base stations repeat the 4 Tframe. Therefore, all channel switching operations of the base stationscan be completed within the 6 T free time which is available duringrepetition of the 8 T frame twice.

Assume that a new channel B3 is designated for a mobile station which ismoving by using a channel A1 at present. In this case, the mobilestation switches the present channel to the channel B3, performs acommunication test with the base station B, and then returns to thechannel A1 when the first half of the continuous 2 T time slots of thechannel A1 is completed. When the communication test is not completedwithin the time slot, the communication test can be performed using aplurality of time slots. With this arrangement, the mobile station cancontinuously perform communication via the channel A1 and at the sametime the communication test of the newly designated channel B3. Thecommunication test for other channels than the channel B3 can beperformed in the same manner as described above.

FIG. 3 is a view showing control procedures of a first handoff sequenceusing the frame formats shown in FIGS. 2A to 2C. When a mobile stationconnected to the base station A via the channel A1 enters into a cellsite of base station B, this entrance is detected by an exchange. Theexchange then selects a free channel B3 of the base station B as a newchannel. This detection technique is well known to those skilled in theart. The exchange designates the channel number B3 of the base station Bto the mobile station via the base station A by using a signal in thechannel A1. The exchange renders the base station B to transmit a testsignal via the channel B3. The mobile station receives the channel B3designated by utilizing the free time of the channel. The mobile stationdetects the test signal and sends back an acknowledge signal via thechannel B3. The exchange which receives the acknowledge signal via thebase station B designates switching to the mobile station by using thesignal in the channel A1. Upon designation of switching, the mobilestation switches the channel from A1 to B3. In this manner, when thecommunication test is performed by utilizing the free time of thechannel, the interruption time of communication can be shortened.

FIG. 4 is a view showing control procedures of a second handoff sequenceusing the channel frame formats shown in FIGS. 2A to 2C. When a mobilestation connected to the base station A via the channel A1 enters into acell site of the base station B, this mobile station detects entranceinto the cell site of the base station B. This detection can beperformed such that a control channel commonly and time-divisionallyutilized by a plurality of base stations is monitored by using thechannel free time and mean values of reception levels of the controlchannel transmitted from the respective base stations are compared. Themobile station which detects crossing between the cell sites requests achannel to the base station B. In response to this, the base station Bdesignates the channel number B3 to the mobile station. The request andthe designation of the channel described above are performed byutilizing the channel free time, and therefore communication is notinterfered. The mobile station then transmits a test signal by utilizingthe free time of the channel A1 via the newly designated channel B3.When the base station B detects this test signal, it sends back anacknowledge signal via the channel B3. When the mobile station receivesthe acknowledge signal from the base station B, the mobile stationdesignates switching to the exchange via the base station A by using asignal in the channel A1. The mobile station then switches the channelfrom A1 to B3. The exchange which receives a signal for designatingswitching switches a communication line from the base station A to thebase station B.

FIG. 5 is a block diagram showing a mobile station for realizing thehandoff sequences of FIGS. 3 and 4. Reference numeral 10 denotes aswitching circuit; 20, a communication circuit; and 30, a communicationtest circuit. An RF signal received by an antenna 100 is converted intoan IF signal by a mixer 11 and demodulated by a modulator/demodulator40. The demodulated signal is input to a time division multiplexer 21.The time division multiplexer 21 extracts a channel signal of presentcommunication from outputs from the modulator/demodulator 40, and theextracted signal is sent to an encoder 22. The encoder 22 extracts acontrol signal which is then supplied to a control circuit 13. Theencoder 22 converts an output from the time division multiplexer 21 intoa speech signal and sends it to a telephone set 23.

A speech signal as an output from the telephone set 23 is converted intoa digital signal by the encoder 22, and the digital signal is input tothe time division multiplexer 21. A control signal as an output from thecontrol circuit 13 constituted by a microprocessor is input to the timedivision multiplexer 21. These signals are converted into a timedivision multiplexed signal by the time division multiplexer 21. Themultiplexed signal is modulated by the modulator/demodulator 40. Themodulated signal is converted into an RF signal by the mixer 11. The RFsignal is transmitted from the antenna 100. Time-division multiplexingand modulation/demodulation techniques are not limited to any specificones.

Upon detection of the signal for designating the new channel number,when the present channel time slot is completed, the control circuit 13changes a frequency of an oscillator 12 to tune the mixer 11 to afrequency of the new channel thereby to cause the communication testcircuit 30 to connect with the new channel through the time divisionmultiplexer 21. When the communication test circuit 30 detects a testsignal via the new channel, it sends back an acknowledge signal.Alternatively, the communication test circuit 30 transmits a test signalvia the new channel and receives an acknowledge signal. When the timeslots of the new channel are completed, the control circuit 13 changesthe frequency of the oscillator 12 to tune to the frequency of thepresent channel and connects the communication circuit 20 to the presentchannel via the time division multiplexer 21 again. With the aboveoperation, the communication test can be performed without adverselyaffecting communication. Therefore, the interruption time ofcommunication during the handoff sequence can be shortened.

Since a conventional mobile telephone system employs analog modulation,an SCPC (Single Channel Per Carrier) method is generally employed inwhich one channel is assigned to one carrier frequency. In the SCPCmethod, since the mobile station is always kept in the transmission orreception state during communication, two receiver/transmitters arerequired to perform a communication test while communication is keptcontinued.

However, when digital modulation is performed, N channels can beassigned to one carrier frequency by time division multiplexing. In thiscase, a time required for setting each mobile station in thetransmission and reception states can be 1/N of the total communicationtime. The mobile station is kept in the standby state during theremaining time. According to the present invention, this free time isutilized to perform a communication test of a new channel by onereceiver/transmitter without interfering the present communication.

In general, in time division multiplexing, the time slots of the channelhave equal intervals. When a new channel which overlaps the presentchannel along the time axis is designated, the time slots of the presentchannel always overlap those of the new channel. Therefore, acommunication test cannot be performed even if a free time is utilized.According to the present invention, the time slots of the channels haveunequal intervals. Even if any channel is newly assigned, a free time isavailable which does not involve overlapping between the time slots ofthe present and new channels. With this time slot assignment, acommunication test can be performed without adversely affectingcommunication, and the interruption time can be almost eliminated.

FIGS. 6A, 6B, 7A, and 7B show channel frame formats according to anotherembodiment of a handoff method of the present invention.

FIGS. 6A and 6B show frame formats of a downward channel from a basestation A to a mobile station and an upward channel from the mobilestation to the base station A. FIGS. 7A and 7B show frame formats of adownward channel from a base station B to a mobile station and an upwardchannel from the mobile station to the base station B.

The downward and upward channels of each base station have apredetermined frequency difference. An 8 T frame constituted by eighttime slots each having a length of T are repeated to constitute eachchannel.

In addition, a time difference between the downward and upward channelsis one time slot. Therefore, after the mobile station receivesinformation in a time slot of the downward channel, it transmitsinformation in a time slot of the upward channel.

Assume that a mobile station connected to the base station A via achannel A1 performs a communication test with the base station B duringcommunication. After the mobile station receives information via theupward channel A1, it changes a frequency to that of the base station Band waits for a test signal via a new downward channel. When the mobilestation detects a test signal via a new downward channel, it sends backan acknowledge signal via the new upward channel, immediately changesthe channel to the channel of the base station A, and startscommunication.

Frequency switching generally requires a given period of time.Accordingly, when the mobile station transmits the signal via the upwardchannel A1 and then changes the frequency, the signal cannot be receivedvia the downward channel B3 but by the channel B4. If newly assignedchannels are B4, B5, and B6, the mobile station detects a test signalvia a downward channel and sends back an acknowledge signal via anupward channel. The mobile station then switches the frequency to thatof the base station A. In this case, information in the next time slotof the channel A1 can be received, and therefore no interruption causedby the communication test occurs.

If newly assigned channels are B7, B8, B1, B2, and B3, the mobilestation detects a test signal via a downward channel and sends back anacknowledge signal via an upward channel. The mobile station thenswitches the frequency to that of the channel of the base station A. Inthis case, the mobile station cannot receive information in the nexttime slot of the channel A1. Therefore, one time slot of the channel A1is lost and interruption occurs. However, such interruption has almostno effect on the communication quality and is negligible.

In the above frame formats, a new downward channel can be detectedwithin one frame upon frequency switching. However, even if a newdownward channel is not detected upon a lapse of one frame, the presentchannel is forcibly restored, and a loss of the time slots can bereduced to one, thereby preventing degradation of communication quality.

In addition, if a loss of a plurality of time slots assures allowablecommunication quality, a new downward channel can be observed for aplurality of frames.

Two handoff sequences according to the channel frame formats shown inFIGS. 6A to 7B will be described with reference to flow charts of FIGS.8 and 9, respectively.

In the handoff sequence of FIG. 8, a mobile station is connected to thebase station A and repeats downward channel reception (step 81) via thechannel A1 and transmission via the upward channel (step 82) of thechannel A1 of the frame formats of FIGS. 6A, 6B, 7A, and 7B. When thismobile station enters into a cell site of the base station B, theexchange detects this entrance and selects a free channel, e.g., B3 ofthe base station B as a new channel. The exchange designates the newchannel to the mobile station by using a signal in the channel A1 viathe base station A.

The exchange causes the base station B to transmit a test signal via thechannel B3. In step 83, upon designation of the new channel, the mobilestation tunes to the carrier of the downward channel B3 at the end ofthe time slots of the channel A1 in step 85. In step 86, a timer is setat one frame period. The mobile station then waits for a test signal viathe downward channel B3. When the mobile station detects the test signalin step 87, the mobile station transmits an acknowledge signal via thenew upward channel B3 in step 89. The mobile station returns to thechannel A1 and continues communication.

If the test signal is not detected upon a lapse of a one-frame period instep 88, the flow returns to step 81. In this case, no operation isperformed, and the mobile station continues communication via thechannel A1. When the exchange receives the acknowledge signal via thebase station B, the exchange designates switching (handoff) to themobile station by using the signal in the channel A1. The exchangeitself switches the channel from A1 to B3. In response to thedesignation of the handoff (step 84), the mobile station switches thechannel from A1 to B3 (step 90). In this manner, when the communicationtest is performed while communication continues, an interruption timeduring the handoff sequence can be shortened.

In the handoff sequence shown in FIG. 9, the mobile station communicateswith the base station via a channel and at the same time receives acommon downward control channel time-divisionally utilized by aplurality of base stations by using a channel free time (step 91). Themean values of the reception levels of the downward control channel ofthe base stations are compared to detect whether the mobile stationenters into another cell site (step 92). For example, when a mobilestation connected to the base station A via the channel A1 enters into acell site of the base station B, the mobile station which has detectedthe entrance in the cell site in step 95 transmits a signal forrequesting a new channel to the base station B via the control channel.In response to this, the base station B designates the channel number B3to the mobile station via the downward control channel and transmits atest signal via the channel B3. Exchange of signals between the mobilestation and the base station B in steps 96 and 97 is performed byutilizing a free time of the channel. The mobile station which isdesignated with the new channel B3 in step 98 tunes the frequency to thecarrier of the channel B3 (step 102) upon completion of the time slotsof the channel A1. The timer is set at one frame period (step 103). Themobile station then waits for a test signal via the channel B3. When themobile station detects the test signal in step 104, the mobile stationtransmits an acknowledge signal via the channel B3 in step 101. The flowreturns to step 96. The mobile station then continues communication viathe channel A1.

However, if the test signal is not detected upon a lapse of a one-frameperiod in step 105, the flow returns to step 91. No operation isperformed, and the mobile station continues communication via thechannel A1. When the base station B receives the acknowledge signal, thebase station B transmits a switching signal to the exchange and themobile station. The exchange and the mobile station which receive theswitching signal in step 99 switch the channel from A1 to B3 in step106.

FIG. 10 is a block diagram of a mobile station for realizing the handoffsequence shown in FIG. 9. Reference numeral 80 denotes a communicationcircuit; 30, a communication test circuit; and 90, a switching circuit.

An RF signal received by an antenna 100 is converted into an IF signalby a mixer 40. The IF signal is demodulated by a modulator/demodulator50. The demodulated signal is input to a time division multiplexer 60.The time division multiplexer 60 extracts a speech signal from theoutput from the modulator/demodulator 50. The speech signal is sent to aswitch 91. The time division multiplexer 60 extracts a control signalfrom the output from the modulator/demodulator 50 and sends it to acontrol circuit 92. The switch 91 is controlled by the control circuit92 to connect the time division multiplexer 60 to an encoder 81 or thecommunication test circuit 30. The time division multiplexer 60 isnormally connected to the encoder 81, and the encoder 81 converts anoutput from the time division multiplexer 60 into a speech signal. Thespeech signal is sent to a telephone set 82.

A speech signal as an output from the telephone set 82 is converted intoa digital signal by the encoder 81. This digital signal is input to thetime division multiplexer 60. A control signal as an output from thecontrol circuit 92 is input to the time division multiplexer 60. Theseinput signals are multiplexed by the time division multiplexer 60, andthe multiplexed signal is the modulator/demodulator 50. The modulatedsignal is converted into an RF signal by the mixer 40. The RF signal isthen transmitted from the antenna 100. Time-division multiplexing andmodulation/demodulation techniques are not limited to any specific ones,and a detailed description will be omitted.

Upon detection of the signal for designating a new channel number, whenthe time slots of the present channel are completed, the control circuit92 changes a frequency of an oscillator 70 and tunes to the frequency ofthe new channel. The switch 91 connects the time division multiplexer 60to the communication test circuit 30, and a timer 93 is set at one frameperiod. When a one-frame period has elapsed, the timer 93 outputs adetection signal to the control circuit 92.

When the communication test circuit 30 detects a test signal via the newchannel, it sends back or transmits an acknowledge signal to signal theend of communication test to the control circuit 92. When thecommunication test is completed, or a one-frame period has elapsed, thecontrol circuit 92 changes the frequency of the oscillator 70 and tunesit to the frequency of the present channel. The time divisionmultiplexer 60 is connected again to the encoder 81, and communicationcontinues. With the above operations, the communication test can beperformed without adversely affecting communication. Therefore, acommunication interruption time during the handoff sequence can beshortened.

According to the above embodiment as has been described above, thecommunication test of the new communication channel can be performedwhile communication continues. Therefore, a new channel can be switchedduring communication without degrading communication quality.

FIG. 11 is a view showing a cellular digital mobile communication systememploying a handoff method according to still another embodiment of thepresent invention.

Referring to FIG. 11, the cellular digital mobile communication systemis constituted by an exchange 200, base stations 220, and a mobilestation 230. The base station 220 includes at least one TDMA radiotransmitter/receiver, and the mobile station 230 includes one TDMA radiotransmitter/receiver.

The mobile station 230 located within the cell site of a given basestation 220 (to be referred to as a BS-A hereinafter) sets acommunication channel with the BS-A and is wired via the exchange 200.When the mobile station 230 is moved from the cell site of the BS-A to acell site of an adjacent base station (e.g., BS-B), the mobile station230 must detect that a new channel can be set with the BS-B. The mobilestation 230 switches the channel with the BS-A into that with the BS-Band continues communication.

FIGS. 12 and 13 show first example of time division of carrierfrequencies and spatial allocation of the time-divisionally carrierfrequencies, respectively.

Referring to FIGS. 12 and 13, the carrier frequencies to be used aregiven as f₁, f₂, f₃, . . . f_(n), and twelve channels aretime-divisionally multiplexed so as to synchronize with the carrierfrequencies f₁, f₂, f₃, . . . f_(n). Channel groups T₁ to T₁₂ areconstituted by channels at identical timings. The channel groups T₁ toT₁₂ are assigned to the base stations such that the channel groups arenot used in the adjacent base stations. In this case, the number of cellsites in a group having an identical frequency is 12.

Since the channel group T₁ is assigned to the BS-A, the mobile station230 located in the cell site of the BS-A performs transmission orreception at the timing T₁ of each of the carrier frequencies f₁ tof_(n). With the this arrangement, when the mobile station 230 which usesthe channel of the channel group T₁ is moved to the cell site of anadjacent base station (one of BS-B to BS-G), the channel groups whichmay be newly assigned to this mobile station 230 are T₄, T₆, T₇, T₉,T₁₀, and T₁₁. Since these channel groups have differences of one or moretime slots from the channel group T₁ which has been used, respectively,a channel group newly assigned to the free time can be designated whilecommunication continues via the channel group T₁, and therefore acommunication can be performed via the new channel group.

FIG. 14 is a view showing a handoff sequence executed according tocarrier frequency allocation shown in FIGS. 12 and 13.

With reference to FIG. 14, assume that a mobile station using a channelof the frequency f₂ of the channel groups T₁ (to be referred to as achannel (T₁,f₂) hereinafter) of the BS-A is moved to a cell site of theBS-B. When this mobile station is moved in the cell site of the BS-B,the exchange detects this movement and selects a free channel, e.g.,(T₆,f₁) as a new channel of the cell site of the BS-B, and designatesthis to the mobile station by using a signal in the channel (T₁,f₂).This detection by the exchange is well known to those skilled in theart, and a detailed description thereof will be omitted. The exchangetransmits a test signal to the BS-B via the channel (T₆,f₁). The mobilestation designated with the new channel (T₆,f₁) waits for a test signaltransmitted via the channel (T₆,f₁) upon completion of the time slots ofthe channel (T₁,f₂). When the mobile station detects the test signal, ittransmits or sends back an acknowledge signal via the channel (T₆,f₁),returns to the channel (T₁,f₂), and continues communication. Acommunication test is performed through two frames in FIG. 14. However,a communication test may be completed within one frame or continued forthree or more frames. When the exchange receives the acknowledge signalvia the channel (T₆,f₁) of the BS-B, the exchange designates switchingto the mobile station by using a signal in the channel (T₁,f₂). Theexchange itself switches the communication line from the BS-A to theBS-B. Upon designation of a handoff sequence, the mobile stationswitches the channel from (T₁,f₂) to (T₆,f₁). In this manner, when thecommunication test is performed while communication continues, aninterruption time during the handoff sequence can be shortened.

FIGS. 15 and 16 are views showing a second example for time division ofcarrier frequencies and spatial allocation of the frequency-dividedcarrier frequencies.

Referring to FIGS. 15 and 16, carrier frequencies to be used are givenas f₁₁, f₁₂, f₁₃, f₂₁, f₂₂, f₂₃, . . . f_(k) l, (k: an integersatisfying l≦k≦n; l=1, 2, 3), . . . f_(n1), f_(n2), and f_(n3), and thechannels are time-divisionally multiplexed to synchronize with the abovecarrier frequencies, thereby constituting channel groups T₁, T₂, . . .T₁₂. The same time slots as the channel T_(i) (i: an integer satisfyingl≦i≦12) are given as S_(i) and R_(i). For example, S₂ represents thatthe carrier frequencies f₁₂, f₂₂, . . . f_(k2), . . . f_(n2) aretime-divisionally multiplexed at the same time slots as in the channelT₂.

In this manner, the channels at identical timings constitute channelgroups T₁ to T₁₂, S₁ to S₁₂, and R₁ to R₁₂. The channel groups T₁ toT₁₂, S₁ to S₁₂, and R₁ to R₁₂ are assigned to the cell sites, as shownin FIG. 15. In this case, clusters each having a size of 12 are combinedto constitute a cluster having a size of 36. Therefore, the number ofcell sites at an identical frequency is 36. Similarly, it is possible toincrease the number of cell sites at an identical frequency while thedegree of multiplexing of channels is kept at 12.

According to this embodiment as described above, a plurality of channelsare time-divisionally multiplexed so as to synchronize with the carrierfrequencies, the channels at identical timings constitute channelgroups, the channel groups are assigned to the cell sites such that thechannel group is used as a unit, and the channel groups continuous onthe time axis are not used by the adjacent cell sites. With thisarrangement, a difference of one or more channels occurs between thepresent channel and the new channel of the cell site adjacent to thepresent channel. Therefore, a communication test of the new channel canbe performed while communication continues via the present channel.Therefore, an interruption time during the handoff sequence can beeliminated.

According to this embodiment as has been described above, there isprovided a handoff method in a cellular digital mobile communicationsystem, wherein transmission timings of the TDMA radiotransmitters/receivers are equal to each other within a given basestation but are differentiated by one or more time slots between theadjacent base stations, so that an interruption time during the handoffsequence can be substantially eliminated while communication continues.

FIG. 17 is a block diagram showing a cellular mobile communicationsystem for practicing a handoff method according to still anotherembodiment of the present invention. Referring to FIG. 17, the cellularmobile communication system is constituted by an exchange 110, N basestations 120₁, 120₂, . . . 120_(N), and a mobile station 130. The N basestations 120₁ to 120_(N) communicate with the mobile station 130 viacommunication and control channels uniquely assigned to these basestations. A downward signal of each control channel of the base stationincludes a flag 140 representing the presence/absence of a free channelof the base station. When the flag 140 is set at level "1", it indicatesthat a free channel is available. However, when the flag 140 is set atlevel "0", it indicates that a free channel is not available. Specificfrequencies are assigned to the communication and control channels so asnot to interfere with each other. Referring to FIG. 17, a frequencyf_(d) is assigned to the downward communication channel of the basestation 120₃, and a frequency f_(u) is assigned to the upward channelthereof. In addition, I frequencies are utilized again for the downwardcontrol channels. Frequencies f₁, f₂, f₃, and f_(I) are assigned to thebase stations 120₁, 120₂, and 120₃, and 120_(I), respectively. Otherfrequency assignment operations are omitted. The mobile station 130comprises an antenna 131, a hybrid circuit 132, a transmitter 133, areceiver 134, a level monitor 135, and a controller 136. The mobilestation 130 communicates with a nearest base station by using thetransmitter 133 and the receiver 134. The mobile station 130 receivesthe signals of the frequencies f₁ to f_(I) via the downward controlchannels of the base stations by using the level motor 135 uponswitching. The mobile station 130 compares reception levels of the basestations having free communication channels and monitors movementbetween the cell sites based on the flags 140. When a change in cellsite is detected, the mobile station 130 transmits a switching requestsignal representing the base station in the changed cell site to theexchange via the upward communication channel.

Assume that the mobile station 130 connected to the base station 120₃via a downward communication channel of the frequency f_(d) and anupward communication channel of the frequency f_(u) is moved from thecell site of the base station 120₃ to the cell site of the base station120₂ having a free communication channel. During travel of the mobilestation 130, the level monitor 135 has a highest reception level of thefrequency f₂ in place of the frequency f₃. The controller 136 comparesthe reception levels of the base stations having free communicationchannels on the basis of the flags 140 and detects a change in cell siteto that of the base station 120₂ and transmits via the upwardcommunication channel a switching request signal for requestingswitching to the base station 120₂. When the base station 120₃ receivesthe switching request signal, it signals this to the exchange 110. Theexchange 110 selects a free communication channel of the base station120₂ and designates its number to the mobile station 130 via the basestation 120₃. The mobile station 130 switches the channel to thedesignated communication channel of the base station 120₂. In thismanner, the mobile station can always monitor a change in cell site, andcontrol of the handoff sequence is shared by the base and mobilestations, thereby preventing local concentration of a control amount. Inthis embodiment, different frequencies are assigned to the controlchannels. However, the control channels may be assigned to an identicalfrequency by time-divisional multiplexing. In this case, the receptionfrequency of the level monitor need not be changed.

FIG. 18 is a block diagram of the mobile station 130 shown in FIG. 17.The mobile station 130 comprises an antenna 131, a hybrid circuit 132, amixer 331, a modulator 332, a mixer 341, a demodulator 342, a mixer 351,a demodulator 352, a level detector 353, an oscillator 350, anoscillator 330, a controller 136, and a telephone set 137. In this case,the mixer 331 and the modulator 332 constitute the transmitter 133. Themixer 341 and the demodulator 342 constitute the receiver 134. The mixer351, the demodulator 352, the level detector 353, and the oscillator 350constitute the level monitor 135. The oscillator 330 is used to controlthe mixers 331 and 342. An RF signal received by the antenna 131 isinput to the mixers 341 and 351 through the hybrid circuit 132. The RFsignal of the downward communication channel is mixed with an outputfrom the oscillator 330 by the mixer 341 and is converted into an IFsignal. The IF signal is demodulated by the demodulator 342. Thedemodulated signal is supplied to the telephone set 137. A controlsignal included in the communication channel is sent to the controller136. A speech signal output from the telephone set 137 and a controlsignal output from the controller 136 are converted into an IF signal bythe modulator 332. An output from the modulator 332 is mixed with anoutput from the oscillator 330 by the mixer 331. An output from themixer 331 is transmitted as an RF signal from the antenna 131 via thehybrid circuit 132. Any modulation/demodulation techniques may beutilized, and a detailed description thereof will be omitted. Onefrequency may be assigned to one communication channel, or one frequencymay be assigned to a plurality of communication channels by timedivision multiplexing.

An RF channel of the downward control channel is mixed with an outputfrom the oscillator 350 by the mixer 351, and the output from the mixer351 is supplied to the demodulator 352 and the level detector 353. Theoutput from the mixer 351 is demodulated by the demodulator 352 and issupplied to the controller 136 as a control signal including a basestation identification code, and a flag representing thepresence/absence of a free communication channel. At the same time, thelevel of the output from the mixer 351 is detected by the level detector353. A level detection signal is supplied to a memory 354. Thecontroller 136 sequentially changes the oscillation frequencies of theoscillator 350 so as to sequentially receive signals via the downwardcontrol channel of each base station. When the controller 136 detectsthe base station identification code in the output from the demodulator352, the controller 136 causes the memory 354 to store the correspondingoutput from the level detector 353 and the flag representing thepresence/absence of the free communication channel in units of basestation identification codes. The controller 136 compares the receptionlevels of the base stations having free communication channels, whichare stored in the memory 354. The control circuit 136 thus monitors achange in cell site. When a change in sell site is detected, thecontroller 136 transmits a switching request signal including thedesignation base station identification code. If a plurality ofreception levels of the respective base stations are stored and theirmean values are compared, the influence of detection errors caused byfading is minimized. In this case, a plurality of flags representing thepresence/absence of free communication channels are stored. Comparisonscan be performed by using the latest flags. In this manner, when achange in cell site is monitored by the mobile station, concentration ofcontrol amounts during the handoff sequence can be prevented.

When the base station monitors a change in cell site upon comparisonbetween the reception levels of the upward signals of the mobilestation, this monitoring must be performed by a plurality of basestations and monitoring results must be concentrated. Therefore, whenchannel switching is frequently performed, the control amount is locallyconcentrated.

According to the present invention, however, each mobile station cancompare the reception levels of the downward signals of the basestations to monitor a change in cell site, so that monitoring controlcan be shared by the base and mobile stations. For this purpose, thelevel monitor is arranged in the mobile station to receive the downwardsignals from other base stations excluding a given station while themobile station communicates with the given station. The downward signalmay be received by the level monitor via a communication channel, butpreferably via a control channel which normally transmits the downwardchannel. When the control channel is time-divisionally multiplexed byeach base station throughout the cell sites, the reception frequency ofthe monitor need not be changed to simplify the apparatus arrangement.The base station transmits a downward signal including the flagrepresenting the presence/absence of a free communication channel, andthe mobile station compares the reception levels of the downward signalsof the base stations having free communication channels. Therefore, abase station having no free communication channel cannot be selected asa designation station. In this manner, the mobile station monitors achange in cell site during communication. When the change in cell siteis detected, the mobile station transmits a switching request signal andis switched to the designation base station having a free communicationchannel. Therefore, local concentration of the control amount during thehandoff sequence can be prevented.

According to this embodiment as has been described above, there isprovided a handoff method and a mobile station in a cellular digitalmobile communication system substantially free from a local increase incontrol amount caused by an increase in the frequency of occurrence ofchannel switching.

What is claimed is:
 1. A mobile station comprising:communicating meansfor performing communication using a time-divisionally multiplexedcommunication channel; switching means for switching a present channelto a new channel during the free time of the time-divisionallymultiplexed communication channel; and a communication test circuit forperforming a communication test of the new channel when said switchingmeans is connected to the new communication channel.
 2. A hand offmethod for a cellular digital mobile communication system comprising thesteps of:providing a time-divisionally multiplexed communicationchannel; changing a frequency to that of a new communication channel ata time at which communications at time slots of the time-divisionallymultiplexed communication channel are completed; and switching the newcommunication channel back to the time-divisionally multiplexedcommunication channel when one of a communication test of the newcommunication channel is completed and a predetermined period of timehas elapsed, thereby performing the communication test of the newcommunication channel.
 3. A mobile station comprising:communicationmeans for performing communication using a time-divisionally multiplexedcommunication channel; switching means for switching a frequency or thetime-divisionally multiplexed communication channel into a frequency ofa second time-divisionally multiplexed communication channel at a timeat which communications at time slots of the time-divisionallymultiplexed communication channel are completed and for switching fromsaid second time-divisionally multiplexed communication channel to thetime-divisionally multiplexed communication channel when one of acommunication test of said second time-divisionally multiplexedcommunication channel is completed and a predetermined period of timehas elapsed; and a communication test circuit for performing thecommunication test of said second time-divisionally multiplexedcommunication channel while said switching means switches the frequencyof the time-divisionally multiplexed communication channel to that ofsaid second time-divisionally multiplexed communication channel.
 4. Ahandoff method for a cellular digital mobile communication system, saidmethod comprising:providing a plurality of base stations and mobilestations, wherein each base station comprises at least onetime-divisional multiplex access radio transmitter/receiver,transmission/reception timings of the time-divisional multiplex accessradio transmitter/receiver are identical within a given base station butare differentiated in adjacent base stations by one or more time slots,and performing a communication test by a mobile station with a basestation adjacent to a specific base station during the free time betweentime slots in the communication channel of said specific base station,while said mobile station is still maintaining communication with saidspecific base station. .Iadd.5. A mobile assisted handoff method for usein a digital cellular system, comprising the steps of: a) receiving at amobile station a communication signal transmitted from a first basestation in a first time slot of a first frequency; b) transmitting fromsaid mobile station to said first base station a communication signal ina second time slot of a second frequency, said second time slot beingdifferent in time from said first time slot; c) detecting at said mobilestation a signal transmitted from a second base station in a third timeslot of a third frequency, said third time slot being different in timefrom said first and second time slots; and d) responsive to the resultof said detecting step (c), determining whether said mobile stationenters a cell site other than the cell site of said first basestation..Iaddend..Iadd.6. A method for use in a digital cellular systemincluding a plurality of mobile stations and a plurality of basestations, comprising the steps of:a) receiving at a first one of saidplurality of mobile stations a communication signal transmitted from afirst one of said plurality of base stations in a first time slot of afirst time-divisionally multiplexed frequency; b) transmitting from saidfirst mobile station to said first base station a communication signalin a second time slot of a second time-divisionally multiplexedfrequency, said second time slot being different in time from said firsttime slot; c) detecting at said first mobile station a signaltransmitted from a second one of said plurality of base stations in athird time slot of a third time-divisionally multiplexed frequency, saidthird time slot being different in time from said first and second timeslots; and d) responsive to the result of said detecting step (c),determining whether handoff of said mobile station isneeded..Iaddend..Iadd.7. A method as claimed in claim 6, wherein saiddetecting step (c) comprises the steps of: c1) detecting at said firstmobile station the levels of signals transmitted from a plurality ofbase stations including said second base station; c2) calculating themean values of the signal levels detected at step (cl); c3) producingsaid mean values calculated at step (c2) as the result of said detectingstep (c)..Iaddend..Iadd.8. A mobile assisted handoff method for use in adigital cellular system, comprising the steps of:a) receiving at amobile station a communication signal transmitted from a first basestation in a first time slot of a first frequency; b) transmitting fromsaid mobile station to said first base station a communication signal ina second time slot of a second frequency, said second time slot beingdifferent in time from said first time slot; c) detecting at said mobilestation a signal transmitted from a second base station in a third timeslot of a third frequency, said third time slot being different in timefrom said first and second time slots; and d) responsive to the resultof said detecting step (c), determining whether handoff of said mobilestation is needed..Iaddend..Iadd.9. A mobile station comprising: meansfor receiving a communication signal transmitted from a first basestation in a first time slot of a first frequency; means fortransmitting to said first base station a communication signal in asecond time slot of a second frequency, said second time slot beingdifferent in time from said first time slot; and means for detecting asignal transmitted from a second base station in a third time slot of athird frequency, said third time slot being different in time from saidfirst and second time slots..Iaddend..Iadd.10. A handoff method for acellular digital mobile communication system comprising the stepsof:providing a time-divisionally multiplexed communication channel;changing a frequency to that of a new communication channel at a time atwhich communications at time slots of the time-divisionally multiplexedcommunication channel are completed; and switching the new communicationchannel back to the time-divisionally multiplexed communication channelwhen one of a crossover test is completed and a predetermined period oftime has elapsed, thereby performing the crossover test during thechange to the new communication channel..Iaddend..Iadd.11. A mobilestation comprising:communicating means for performing communicationusing a time-divisionally multiplexed communication channel; switchingmeans for switching a present channel to a new channel during the freetime of the time-divisionally multiplexed communication channel; and acrossover test circuit for performing a crossover test by receiving atransmission on said new channel when said switching means is connectedto the new communication channel..Iaddend..Iadd.12. A handoff method fora cellular digital mobile communication system, said method comprising:providing a plurality of base stations and mobile stations, wherein eachbase station comprises at least one time-divisional multiplex accessradio transmitter/receiver, transmission/reception timings of thetime-divisional multiplex access radio transmitter/receiver areidentical within a given base station but are differentiated in adjacentbase stations by one or more time slots, and performing a crossover testby a mobile station with a base station adjacent to a specific basestation during the free time between time slots in the communicationchannel of said specific base station, while said mobile station isstill maintaining communication with said specific basestation..Iaddend..Iadd.13. A mobile station comprising:communicationmeans for performing communication using a time-divisionally multiplexedcommunication channel; switching means for switching a frequency of thetime-divisionally multiplexed communication channel into a frequency ofa second time-divisionally multiplexed communication channel at a timeat which communications at time slots of the time-divisionallymultiplexed communication channel are completed and for switching fromsaid second time-divisionally multiplexed communication channel to thetime-divisionally multiplexed communication channel when one of acrossover test at said second time-divisionally multiplexedcommunication channel is completed and a predetermined period of timehas elapsed; and a crossover test circuit for performing the crossovertest while said switching means switches the frequency of thetime-divisionally multiplexed communication channel to that of saidsecond time-divisionally multiplexed communication channel..Iaddend.